1. Field of the Invention
The present invention relates to a light-absorbing layer for a solar cell and a method of manufacturing the light-absorbing layer. More particularly, the present invention relates to a light-absorbing layer for a solar cell with enhanced sunlight absorption comprising CuInSe2, CuGaSe2 and CuIn1-xGaxSe2 thin films laminated by metal organic chemical vapor deposition (MOCVD), and a method of manufacturing the light-absorbing layer.
2. Description of the Related Art
Ternary thin films, including CuInSe2 (hereinafter, referred to simply as “CIS”) and CuIn1-xGaxSe2 (hereinafter, referred to simply as “CIGS”), are compound semiconductors on which a number of studies have been actively undertaken.
Unlike conventional solar cells employing silicon, CIS-based thin film solar cells can be manufactured to have a thickness not greater than 10 microns and exhibit superior stability even after long-term use. In addition, CIS-based thin film solar cells are experimentally proven to possess a maximum conversion efficiency of 19.8% as compared to conventional solar cells. Accordingly, CIS-based thin film solar cells have attracted commercial interest for replacing silicon solar cells in terms of low price and high efficiency.
For successful commercialization, various methods for forming CIS thin films have been recently reported. For example, details of one method for forming CIS thin films are described in Korean Patent Application No. 2004-29221, which was filed by the present applicant. According to this method, first, an InSe thin film is formed from [Me2In—(μSeMe)]2 as a precursor on a substrate by metal organic chemical vapor deposition, a Cu2Se thin film is formed using (hfac)Cu(DMB) as a precursor on the InSe thin film by metal organic chemical vapor deposition, and then a CuInSe2 thin film is a formed using [Me2In—(μSeMe)]2 as a precursor on the Cu2Se thin film by metal organic chemical vapor deposition. Further, a CuIn1-xGaxSe2 thin film is formed using [Me2Ga—(μSeMe)]2 as a precursor on the CuInSe2 thin film by metal organic chemical vapor deposition.
As described above, CIS-based thin film solar cells show a high-energy conversion efficiency close to 20%, but they should be formed in a multilayer structure in order to further increase the efficiency. Sunlight contains abundant invisible ultraviolet and infrared rays as well as visible rays having a photon energy between about 1.7 eV (i.e. red ray having a wavelength of 700 nm) and 3.0 eV (i.e. violet ray having a wavelength of 400 nm). Consequently, techniques for forming CIS-based thin films into a multilayer structure are required in the fabrication of high-efficiency solar cells for absorbing incoming light with various energies.
To efficiently convert the entire spectrum of sunlight to electrical energy, light-absorbing layers having different bandgap energies should be arranged in a multilayer structure. It is known that the theoretical energy conversion efficiency of solar cells employing CIS is 42% for 2-layer solar cells, 49% for 3-layer solar cells, 53% for 4-layer solar cells, and a maximum of 68% for 5 or more-layer solar cells. In this manner, a cell termed a “tandem cell” can be fabricated by continuously or discontinuously varying the compositions of light-absorbing layers. CIS-based compound thin films are polyatomic materials. When CIS-based compound thin films are formed into a multilayer structure, the constituent atoms are diffused at the interface between upper and lower layers, losing inherent characteristics of the thin films. For this reason, the fabrication of high-quality tandem cells presents considerable difficulties.